1. Field of the Invention
The present invention relates to a conductive paste used for forming internal electrodes of a multilayer ceramic electronic device, a multilayer ceramic electronic device produced using that paste, and a method of production of the device.
2. Description of the Related Art
In recent years, electronic apparatuses have been made thinner and smaller. Along with this, the multilayer ceramic electronic devices used for these electronic apparatuses are also being made smaller in size and higher in capacity.
The most effective method for reducing the size and increasing the capacity of a multilayer ceramic capacitor, explained here as an example of a multilayer ceramic electronic device, is to make both the internal electrodes and dielectric layers as thin as possible and stack as many of them as possible.
A multilayer ceramic capacitor is produced by printing ceramic green sheets having a ceramic powder such as barium titanate and a binder as main ingredients with a conductive paste for forming internal electrodes in predetermined patterns, stacking them, then simultaneously firing them to sinter them and then finally forming external electrodes.
As the conductive paste for forming the internal electrodes, one comprised of an organic binder dissolved in a solvent to form an organic vehicle into which a conductive powder is then dispersed is used. As the organic binder in the organic vehicle, for example ethyl cellulose etc. is used, while as the solvent in the organic vehicle, terpineol etc. is used.
However, if printing a conductive paste using terpineol as a solvent on a ceramic green sheet, the problem of seepage etc. occurred and electrode patterns accurately reduced to a predetermined thickness cannot be obtained.
Further, when using a conductive paste using terpineol as a solvent in combination with a ceramic green sheet using a butyral resin as an organic binder, the solvent in the conductive paste makes the organic binder in the ceramic green sheet swell or dissolve, i.e., the so-called “sheet attack” phenomenon occurs.
This sheet attack phenomenon does not become a practical problem while the ceramic green sheet is relatively thick. However, when the thickness of a ceramic green sheet is for example 5 μm or less, if the sheet attack phenomenon occurs, when printing the conductive paste, then peeling off the ceramic green sheet from a polyethylene terephthalate (PET) film or other carrier sheet, the ceramic green sheet becomes hard to peel off. If the ceramic green sheet becomes hard to peel off, this results in the ceramic green sheet wrinkling, being punctured, cracking, etc. and makes it impossible to obtain a normal stack in the stacking process. If a normal stack cannot be obtained, the finally obtained multilayer ceramic electronic device suffers from short-circuit defects, withstand voltage defects (IR deterioration), and the phenomenon of peeling between the dielectric layers and internal electrode layers (delamination) which in turn cause a drop in yield.
Therefore, in recent years, several means have been proposed for dealing with this sheet attack phenomenon. As a solvent which will not dissolve a butyral resin, Japanese Patent Publication (A) No. 9-17687 proposes to use dihydroterpineol, while Japanese Patent No. 2976268 proposes a conductive paste using dihydroterpineol acetate.
With dihydroterpineol or dihydroterpineol acetate, the solubility of the ethyl cellulose used as the organic binder is relatively good. Therefore, these might be effective for the improvement of the solubility.
However, the conductive pastes described in Japanese Patent Publication (A) No. 9-17687 and Japanese Patent No. 2976268 using dihydroterpineol or dihydroterpineol acetate as a solvent have the problem of a tendency to increase in viscosity along with time. Therefore, while the viscosity is as desired and the pastes can be formed on ceramic green sheets to a predetermined thickness at the start of printing, after the elapse of a predetermined time, the viscosity increases and the same thickness cannot be formed under the printing conditions of the start of printing. This problem tends to manifest itself when desiring to form the electrode patterns particularly thin.
Therefore, with such conventional conductive pastes, there were limits to the reduction of size and increase in capacity of multilayer ceramic electronic devices.
Note that Japanese Patent Publication (A) No. 2002-270456 proposes to suppress changes in viscosity of a conductive paste along with time by using as the solvent of the conductive paste isobonyl acetate or nopyl acetate.